1. Field of the Invention
The present invention relates to a method for examining the pulmonary mechanics of a respiratory system.
The present invention also relates to a breathing apparatus system operating according to the method.
2. Description of the Prior Art
A ventilator is an apparatus for ventilating or supporting spontaneous ventilation of the lungs of a human being or an animal, hereafter referred to as the xe2x80x98patientxe2x80x99. The procedure generally employed for tailoring a ventilator""s operation to physiological conditions prevailing in the patient is for the responsible care-provider to obtain as much information as possible about the patient, information about the respiratory organs in particular. This type of information often can be obtained when the ventilator is equipped with sensors for airway pressure, flow or volume delivered to or removed from the respiratory organs. Measurement systems, however, often are formed of one or a number of devices independent of the ventilator. The measurement systems can be linked to systems for analysing pulmonary function. Thus, measurement signals for pressure, volume and flow can be deployed so as to yield information about the elasticity of the respiratory organs, referred to below as compliance, or about resistance to flow in the airways. Ventilators can contain and/or be connected to computers that read the signals from sensors and analyse them. Computers or electronic circuits incorporated in the respirator, hereafter referred to as xe2x80x98the computerxe2x80x99, act on the ventilator""s functional modes and cause the ventilator to perform test breaths yielding increased information about pulmonary function. Test breaths of this kind can mean that the computer controls both inspiration and expiration for obtaining the desired physiological information.
One example of such control is disclosed in Swedish Patent No. 506521, which describes control whereby expiration is affected in such a way that its duration is prolonged, and a constant target airway pressure is maintained. This expiration is followed by an inspiration during which the flow is modulated, e.g. with a sinusoidal waveform. The variation in flow during inspiration leads to corresponding variations in airway pressure. The relationship between variations in pressure and flow reflects resistance in the respiratory system. This resistance can be calculated. At the same time, the respiratory system""s elastic rebound pressure can be calculated in relation to the insufflated volume. This yields the respiratory system""s elastic pressure/volume curve, i.e. the Pell/V curve. The Pell/V curve is believed to supply information about the positive end-expiratory pressure (PEEP) required to keep open the lungs of a patient with acute respiratory distress syndrome (ARDS). This has been questioned, however, since the morphology of the Pell/V curve is influenced by complex phenomena and therefore difficult to interpret. Multiple Pell/V curves recorded at different levels of PEEP can supply more detailed information, but recording these curves is difficult and time-consuming.
For some time, it has been known that the recording of both inspiratory and expiratory pressure-volume curves, thereby yielding a pressure-volume loop, supplies information about the forces required to open a collapsed lung. A significant difference between the inspiratory curve and the expiratory curve gives the loop a large area referred to as large hysteresis. The greater the hysteresis, the greater the tendency of the lung to collapse and resist re-expansion, so-called recruitment. This means that internal forces in the lung are particularly strong during recruitment, and this poses a great risk of respirator-induced lung damage (RILS).
Elastic pressure-volume curves for inspiration and expiration, together with the Pel/V loops, were previously recorded during respirator treatment using a xe2x80x98jumboxe2x80x99 syringe. This is disclosed in e.g. U.S. Pat. No. 4,844,085. Recording takes a long time. This means that gas exchange in the form of gas uptake leads to artifacts, making accurate determination of hysteresis impossible.
Pel/V loops also can be determined with the flow occlusion method. This determination takes several minutes during which the patient""s condition can change, thereby making accurate determination of hysteresis impossible in clinical settings. No methods are available for accurate, automatic measurement of pressure-volume loops in clinical practice.
An object of the present invention is to provide a method for examining pulmonary mechanics that solves at least some of the aforementioned problems of the known methods.
Another object of the present invention is to provide a respiratory apparatus system for use in the examination of pulmonary mechanics and in the treatment of respiratory systems (in humans and animals). The system must be easy to use in examinations of pulmonary mechanics in clinical settings without affecting ongoing treatment of the respiratory system.
Another object of the present invention is to provide a breathing apparatus system, and a method for the operation thereof, for automatic regulation of respiratory parameters on the basis of the respiratory system""s mechanical condition.
The above object is achieved in accordance with the principles of the present invention in a method for operating a breathing apparatus for examining the pulmonary mechanics of a respiratory system communicating with the breathing apparatus, wherein the flow of gas exiting from the respiratory system is modulated, the volume of the gas exiting the respiratory system is determined, the variation in pressure in the respiratory system is determined, and an expiratory pressure-volume relationship is determined from the aforementioned volume and the aforementioned pressure.
The elastic properties of the respiratory system can be established in a simpler fashion during expiration by modulating the flow of gas streaming out of the respiratory system. Volume and pressure are determined in relation to time during expiration and can then be used for determining an expiratory pressure-volume relationship. The relationship obtained refers advantageously to the elastic part of the ratio, i.e. the resistive components have been subtracted.
Modulation can be sinusoidal, triangular, quadratic or have some other regular shape. Declining or increasing modulation with a regular basic form or even completely irregular modulation can be used. Pressure can be measured inside the respiratory system, e.g. in the airways, or at some other location in the path of gas from the respiratory system. When pressure is not measured in the respiratory system itself, the corresponding pressure can be calculated from the measured pressure by compensating for the pressure drop in the system of tubing etc. Measuring the flow and integrating it determine the volume most easily. However, other ways of determining volume are possible.
The expiratory pressure-volume relationship (which can be represented by a curve in a system of co-ordinates) can be determined in the corresponding manner by modulating flow during inspiration. Comparison supplies a measure of hysteresis. Hysteresis can then be used for evaluations in the known manner.
Repeating the method for a number of breaths would be advantageous. A number of normal breaths should then be allowed between these test breaths. This is especially the case when the pressure-volume relationship for inspiration is established after a preceding expiration with a prolonged duration.
The above object also is achieved in accordance with the present invention in a breathing apparatus for use in examination and treatment of the pulmonary mechanics of a respiratory system, having a pneumatic unit for regulating the flow and pressure of gases, a measurement sensor system for measuring pressure and flow, a control unit connected to the pneumatic unit for control thereof and to the measurement sensor system to receive measurement signals therefrom, and a tubing system connected to the pneumatic unit and connectible to the respiratory system, wherein the control unit controls the pneumatic unit during expiration so that the pneumatic unit modulates a flow of gas exiting from the respiratory system, and wherein the control unit determines an expiratory pressure-volume relationship from measurement signals from the measurement sensor system.
In principle, the control unit in the breathing apparatus system is devised to permit control of test breaths according to the described method.
The control unit also can be devised to perform automatic changes in certain parameters on the basis of results obtained for hysteresis. PEEP, tidal volume and frequency in particular can be parameters for automatic control.